The story follows Harold, a curious and adventurous dog, and his best friend, Chester, a wise and grumpy cat. Their owners, the Monroe family, adopt a strange, white rabbit from a pet store, naming him Bunnicula.
Harold and Chester quickly notice that Bunnicula acts differently than other rabbits. He has an unusual fear of sunlight, sleeps upside down, and drains the color from vegetables, leaving them white.
The novel follows Harold and Chester's attempts to uncover Bunnicula's true nature, while the Monroes remain oblivious to the paranormal happenings in their home. The story is filled with humor, suspense, and a touch of the macabre, as the animals investigate the strange events and try to protect their home from Bunnicula's mysterious powers.
Ultimately, the novel explores themes of friendship, courage, and the power of believing in the unknown.